Catalytic reforming is one of the basic petroleum refining processes for upgrading light hydrocarbon feedstocks, frequently referred to as naphtha feedstocks. Products from catalytic reforming can include high octane gasoline useful as automobile fuel, aromatics (for example benzene, toluene, xylenes and ethylbenzene), and/or hydrogen. Reactions typically involved in catalytic reforming include dehydrocylization, isomerization and dehydrogenation of naphtha range hydrocarbons, with dehydrocyclization and dehydrogenation of linear and slightly branched alkanes and dehydrogenation of cycloparaffins leading to the production of aromatics. Dealkylation and hydrocracking during catalytic reforming are generally undesirable due to the low value of the resulting light hydrocarbon products.
Xylene is composed of three different isomers, para-xylene (PX), meta-xylene (MX), and ortho-xylene (OX). Of the xylene isomers, para-xylene (PX) is of particular value since it is useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers. One current method for producing para-xylene is using naphtha reforming where mixed aromatics are produced. An aromatic containing stream can be separated and the stream used as a feedstock for the production of para-xylene. Generally, para-xylene is produced along with other xylene isomers and toluene. Purified toluene may be selectively or non-selectively disproportionated to produce para-xylene and benzene. Para-xylene may also be produced from mixed xylenes by isomerization followed by separation of the para-xylene from the meta and ortho isomers.
One known method for producing xylenes involves the alkylation of toluene with methanol over a solid acid catalyst. The alkylation of toluene with methanol over cation-exchanged zeolite Y has been described by, for example, Yashima et al. in the Journal of Catalysis 16, 273-280 (1970). Under optimized reaction conditions, the amount of para-xylene produced was approximately 50 wt % of the xylene product mixture.
U.S. Pat. Nos. 7,119,239 and 7,176,339 disclose a process for the production of xylenes from reformate. The process is carried out by methylating, under conditions effective for the methylation, the benzene/toluene present in the reformate, to produce a resulting product having a higher xylenes content than the reformate. Greater than equilibrium amounts of para-xylene can be produced by the process. U.S. Pat. No. 7,186,873 discloses a process for the production of xylenes from reformate by reactive distillation. The process is carried out by methylating the benzene/toluene present in the reformate in a reactive distillation zone and under reactive distillation conditions to produce a resulting product having a higher xylenes content than the reformate. Greater than equilibrium amounts of para-xylene can be produced by the process.
Given the higher demand for para-xylene as compared with other xylene isomers, there is significant commercial interest in maximizing para-xylene production from any given source of C8 feedstocks. However, there are two major technical challenges in achieving this goal of maximizing para-xylene yield. Firstly, the four C8 aromatic compounds, para-xylene, meta-xylene, ortho-xylene, and ethylbenzene, are usually present in concentrations dictated by thermodynamic equilibria, where meta-xylene comprises about 60 wt. %, para-xylene about 14 wt. %, ortho-xylene about 9 wt. %, and ethylbenzene about 17 wt. % of the C8 aromatic compounds. As a result, the para-xylene yield is limited from any refinery C8 stream unless additional processing steps are used to increase the amount of para-xylene and/or to improve the para-xylene recovery efficiency. Secondly, the C8 aromatics are difficult to separate due to their similar chemical structures and physical properties and identical molecular weights.
A variety of methods are known to increase the concentration of para-xylene in a C8 aromatics product stream. These methods normally involve recycling the product stream between a separation step, in which at least part of the para-xylene is recovered to produce a para-xylene-depleted stream, and a xylene isomerization step, in which the para-xylene content of the para-xylene-depleted stream is returned back towards equilibrium concentration, typically by contact with a molecular sieve catalyst. However, the commercial utility of these methods depends on the efficiency, cost effectiveness and rapidity of the separation step which, as discussed above, is complicated by the chemical and physical similarity of the different C8 isomers.
A variety of methods are known in the art to purify para-xylene from less valuable xylene isomers and ethylbenzene. Fractional distillation is a commonly used method for separating different components in chemical mixtures. However, it is difficult to use conventional fractional distillation technologies to separate ethylbenzene (EB) and the different xylene isomers because the boiling points of the four C8 aromatics fall within a very narrow range, namely from about 136° C. to about 144° C. In particular, the boiling points of para-xylene and EB are about 2° C. apart, whereas the boiling points of para-xylene and meta-xylene are only about 1° C. apart. As a result, large equipment, significant energy consumption, and/or substantial recycles would be required for fractional distillation to provide effective C8 aromatic separation. Another method for separating the para-xylene from other xylene isomers and ethylbenzene involves crystallizing the para-xylene. U.S. Pat. No. 5,811,629 discloses a process for purifying para-xylene from C8 aromatics involving at least two crystallization stages as well as at least one recycle step and at least one additional separation step. The above described methods are time consuming and costly. It is desirable to increase the amount of para-xylene in the product stream so as to minimize the number of recycle and purification steps needed to obtain pure para-xylene product.
It has been found that the use of a low acidity medium pore zeolite catalyst with a silica to alumina ratio of at least about 40 to 1, increases the yield of para-xylene from a given C8 paraffinic feedstock.